This RAND review of the literature analyzed 1,457 studies related to manipulation of the cervical spine. Only sixty-seven of these were used in the final analysis, as they were the only ones that specifically addressed the effectiveness of treatment.
"Analysis of the existing data has led the authors to conclude that 1) mobilization is probably of at least short-term benefit for patients with acute neck pain; 2) manipulation is probably slightly more effective than mobilization or physical therapy for some patients with subacute or chronic neck pain (and all three treatments are probably superior to usual medical care); and 3) manipulation and/or mobilization may be beneficial for muscle tension headache."
In terms of adverse complications as a result of chiropractic manipulation, the authors state, “The complication rate for cervical spine manipulation is estimated to be between 5 and 10 per 10 million manipulations.” [This is a rate between 0.0000005 and 0.000001.]
Hurwitz EL, Aker PD, Adams AH, et al. Manipulation and mobilization of the cervical spine: a systematic review of the literature. Spine 1996;21(15):1746-1760.